Multilevel semiconductor-on-insulator structures and circuits

ABSTRACT

Some advanced integrated circuits are fabricated as silicon-on-insulator structures, which facilitate faster operating speeds, closer component spacing, lower power consumption, and so forth. Unfortunately, current bonded-wafer techniques for making such structures are costly because they waste silicon. Accordingly, one embodiment of the invention provides a smart-bond technique that allows repeated use of a silicon wafer to produce hundreds and potentially thousands of silicon-on-insulator structures, not just one or two as do conventional methods. More precisely, the smart bond technique entails bonding selected first and second regions of a silicon substrate to an insulative substrate and then separating the two substrates to leave silicon protrusions or islands on the insulative substrate. The technique is also suitable to forming three-dimensional integrated circuits, that is, circuits having two or more circuit layers.

This application is a Divisional of U.S. Ser. No. 09/128,851, filed on Aug. 4, 1998, now U.S. Pat. No. 6,093,623.

BACKGROUND OF THE INVENTION

The present invention concerns fabrication methods and structures for integrated circuits, particularly methods for making silicon-on-insulator structures.

Integrated circuits, the key components in thousands of electronic and computer products, are interconnected networks of electrical components fabricated on a common foundation, or substrate. Fabricators typically use various techniques, such as layering, doping, masking, and etching, to build thousands and even millions of microscopic resistors, transistors, and other electrical components on a silicon substrate, typically a round slice of silicon called a wafer. The components are then wired, or interconnected, together to define a specific electric circuit, such as a computer processor or memory.

The evolution of integrated circuits has been driven by three principal objectives: reducing size, lowering power consumption, and increasing operating speed. Silicon-on-insulator (SOI) technology - - - an emerging technology which entails building silicon devices, such as transistors, on an insulative substrate rather than on a silicon substrate as done typically - - - dramatically advances all three objectives. First, silicon-on-insulator technology provides superior electrical isolation between adjacent components, which, in turn, allows closer component spacing and integrated-circuit size reductions. Second, silicon-on-insulator technology reduces integrated-circuit capacitance, the primary obstacle to faster operating speeds. And third, it enables use of lower operating voltages, which significantly reduces power consumption.

Unfortunately, conventional methods for implementing silicon-on-insulator technology waste costly silicon, and therefore have been commercially viable in only a few high-priced applications. For instance, one conventional method forms an insulative layer on the top surface of a silicon wafer and bonds the entire surface of another silicon wafer onto the insulative layer, essentially sandwiching the insulative layer between the two silicon wafers. This method then wastefully grinds down one of the silicon wafers to a thin silicon layer, yielding one silicon-on-insulator structure at the cost of two silicon wafers.

A similar method, dubbed “smart cut” by its proponents, is a bit more cost effective. The smart-cut method sandwiches an insulative layer between two silicon wafers, but rather than grinding down one of the wafers, it in effect slices off a thick portion of one wafer, leaving a thin slice of silicon covering the entire insulative layer and saving the thick portion for use in another silicon-insulator-silicon sandwich. Thus, the smart-cut method is emerging as a substitute to the bond-and-grind method.

However, the smart-cut method suffers from at least three shortcomings. First, slicing the silicon wafer entails implanting hydrogen ions into the silicon, some of which remain in the bonded silicon after the slicing procedure, and introduce defects into transistors later formed in the silicon. Second, the smart-cut method indiscriminantly bonds a continuous silicon layer to the entire insulative surface, not only to regions where transistors are desired but also to regions where they aren't. The continuity of the silicon layer forces it to buckle and crack during subsequent processing because the insulation and silicon expand and contract at very different rates in response to temperature changes. Third, the continuity also allows silicon between and around intended transistors to form unintended, or parasitic, devices, for example parasitic diodes and transistors, which compromise isolation, performance, and reliability of the intended transistors.

Accordingly, there is a need for more effective methods of making silicon-on-insulator structures.

SUMMARY OF THE INVENTION

To address these and other needs, the inventor devised a smart-bond technique that allows repeated use of a silicon or more generally a semiconductive wafer to produce hundreds, potentially thousands, of semiconductor-on-insulator structures. Moreover, the smart-bond technique intelligently applies semiconductive material only where semiconductor devices are desired, not only saving semiconductive material, but precluding formation of parasitic devices which would otherwise compromise isolation, performance, and reliability.

More particularly, the smart-bond technique entails bonding or fusing selected first and second regions of a semiconductive substrate to an insulative substrate and then separating the two substrates to leave semiconductive protrusions or islands on the insulative substrate. To reuse the semiconductive substrate, one repeats the bonding and separation to form-semiconductive protrusions on another insulative substrate. The semiconductive protrusions can be made into transistors which are interconnected to form integrated circuits which are not only smaller, faster, and more efficient but also less costly than conventional integrated circuits with silicon-on-insulator technology.

Other embodiments apply the selective bonding and substrate separation steps two or more times to make an integrated circuit having two or more circuit levels, in other words, a “three-dimensional” integrated circuit. More particularly, this entails bonding selective regions of a semiconductive substrate to a first insulative substrate to form a first set of semiconductive protrusions, which are then made into a first set of interconnected transistors to form a first circuit level. The bonding and separation are then repeated using the same semiconductive substrate to form a second set of semiconductive protrusions on a second insulative substrate positioned over the first set of interconnected transistors. Transistors are formed from the second set of protrusions and subsequently interconnected to complete a second circuit level. Thus, the present invention also offers an efficient method of making three-dimensional integrated circuits with the performance advantages of silicon-on-insulator technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a part of a semiconductive donor substrate 12 having a number of integral semiconductive protrusions 14 for use in fabricating an integrated circuit;

FIG. 2 is a cross-sectional view of donor substrate 12 taken along line 2—2, illustrating two particular semiconductive protrusions 14 a and 14 b, which include respective top portions 15 a and 15 b and respective base portions 16 a and 16 b;

FIG. 3 is a cross-sectional view of an integrated-circuit assembly resulting from bonding or fusing select regions of substrate 12, specifically top portions 15 a and 15 b of protrusions 14 a and 14 b, to an insulative substrate 18;

FIG. 4 is a cross-sectional view of the FIG. 3 assembly after separation of semiconductive substrate 12 from insulative substrate 18 leaves top portions 15 a and 15 b of protrusions 14 a and 14 b bonded to substrate 18;

FIG. 5 is a cross-sectional view of the FIG. 4 assembly after planarizing top portions 15 a and 15 b, forming drain and source regions 20 d and 20 s, and depositing or growing an overlying gate insulation layer 22.

FIG. 6 is a cross-sectional view of the FIG. 5 assembly after forming gates 24 a and 24 b, an interconnecting conductor 25, and an overlying insulative layer 26;

FIG. 7 is a cross-sectional view of the FIG. 6 assembly after forming new protrusions 28 a and 28 b in semiconductive donor substrate 12 and bonding them to insulative layer 26;

FIG. 8 is a cross-sectional view of the FIG. 7 assembly after separating donor substrate 12 from insulative layer 26, forming top portions 29 a and 29 b of protrusions 28 a and 28 b into transistors 29 a′ and 29 b′, and interconnecting them through conductor 35; and

FIG. 9 is a block diagram of a generic integrated memory circuit fabricated in accord with the method and structures of FIGS. 1-8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description, which references and incorporates FIGS. 1-9, describes and illustrates specific embodiments, or versions, of the invention. These embodiments, offered not to limit but only to exemplify and teach, are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art.

FIGS. 1-8 show a number of views and structures, which taken collectively and sequentially, illustrate an exemplary method of forming an integrated circuit incorporating silicon-on-insulator, and more generally semiconductor-on-insulator technology. The method, as shown in FIG. 1, begins with a semiconductive donor substrate 12. (The term “substrate,” as used herein, broadly encompasses any structure or surface, regardless of composition, which may directly or indirectly support all or any portion of an integrated circuit.) Semiconductive donor substrate 12 serves as source of semiconductive material, for example, silicon. However, in other embodiments, donor substrate 12 consists of gallium-arsenic, germanium, silicon-carbide, or combinations of these materials. Thus, the invention is not limited to any particular semiconductive material.

Donor substrate 12 includes a number of semiconductive protrusions or islands 14, which will ultimately be bonded to an insulative substrate to form semiconductor-on-insulator structures. The protrusions are noncontiguous in this embodiment to promote device isolation. FIG. 2, a cross-sectional view of donor substrate 12 taken along line 2—2, particularly shows two semiconductive protrusions 14 a and 14 b, not only as being integral to substrate 12 but also as including respective planar top portions 15 a and 15 b and respective base portions 16 a and 16 b. In this exemplary embodiment, base portions 16 a and 16 b are narrower than corresponding top portions 15 a and 15 b. The top portions are generally square in shape with sides of one micron or less; the bases are generally rounder with a diameter of one-half micron or less; and the overall height of the protrusions, which is one micron or less, matches that of the top side dimensions. However, other embodiments of the invention provide various protrusions ranging from heterogeneous, amorphous outcroppings to highly congruent geometric solids. In fact, in one embodiment of the invention, the protrusions comprise whole transistors or portions of transistors, with appropriate doping and so forth. Thus, the invention encompasses any form, shape, and size of protrusion or island.

Protrusions 14 may be formed using any number of techniques. For example, co-pending and co-assigned patent application Ser. No. 08/745,708, teaches a method of forming silicon protrusions. This application, entitled Silicon-on-Insulator Islands and Method for Their Formation, is incorporated herein by a reference. In addition, U.S. Pat. No. 5,691,230, entitled Technique for Producing Small Islands of Silicon on Insulator, teaches a technique of forming silicon pillars and fully undercutting them with oxide formations to form silicon-on-insulator structures. The exemplary method adapts this technique to form protrusions 14 by only partially undercutting the pillars.

More particularly, this technique starts with a lightly doped silicon wafer, for example, a p-type wafer, grows or deposits 10-to-30 nanometer-thick, oxide layer on the wafer, and then grows or deposits a 100-nanometer thick layer of silicon nitride atop the oxide layer. Afterward, a series of perpendicularly intersecting trenches are patterned in the nitride and oxide layers to define individual island formations, with the trenches being directionally etched to a depth of around 200 nanometers, measured from the top of the nitride layer into the silicon substrate. The technique next deposits or grows another silicon nitride layer, with a thickness less than 100 nanometers, on the top and sides of the protrusions and then directionally etches away the silicon nitride on the tops, leaving silicon nitride on the sides of the protrusions. To form the narrow base portions, one isotropically etches into the silicon substrate, with the silicon nitride in place to protect the protrusion tops.

Lastly, the nitride and oxide are removed to leave protrusions such as those shown in FIGS. 1 and 2.

As FIG. 3 shows, the next step bonds or fuses select regions of semiconductive substrate 12, specifically top portions 15 a and 15 b of protrusions 14 a and 14 b, to an insulative carrier substrate 18. In the exemplary embodiment, insulative carrier substrate 18 is a smooth, polished, and oxidized insulative layer, such as silicon oxide, which has been grown or deposited on a semiconductive wafer. However, other embodiments provide the insulative carrier substrate as a sapphire or quartz structure optically polished to a smooth finish. Pre-bonding preparations also include oxidizing carrier substrate 18 in a high-temperature, that is 800-1100° C., environment and cleaning the donor and carrier substrates in, for example, a standard RCA or hydrophilization clean.

The actual bonding may follow any number of techniques. Bonding in the exemplary embodiment occurs in the same temperature range used for oxidation. More specifically, it entails placing semiconductive donor substrate 12, specifically top portions 15 a and 15 b, in contact with portions of the oxidized surface of insulative carrier substrate 18 for a period of time. Some embodiments apply an optional electrostatic force or other clamping force to facilitate a stronger bond than can be achieved under weight of the semiconductive donor substrate alone. A bond between top portions 15 a and 15 b of donor substrate 12 and contacting portions of the surface of carrier substrate 18 results after about 5 minutes in the high-temperature environment.

After bonding, the semiconductive donor and insulative carrier substrates, which have different thermal-expansion coefficients, are allowed to cool. In the exemplary embodiment, the silicon underlying the insulative layer of the carrier substrate contracts more than the insulative layer, causing carrier substrate 18 to bow and thus to shear, or break, base portions 16 a and 16 b of protrusion 14 a and 14 b in two. One then separates donor substrate 12 and the lower-most base portions of protrusions 14 a and 14 b from carrier substrate 16, leaving top portions 15 a and 15 b of protrusions 14 a and 14 b bonded to carrier substrate 18.

Other embodiments break base portions 16 a and 16 b by heating substrates 12 and 16 above the bonding temperature, again relying on differences in thermal-expansion coefficients of the donor and insulative substrates. Still other embodiments apply a known smart-cut technique which entails hydrogen-ion implantation to create a heat-activatable cleavage plane through the base portions of the protrusions, prior to bonding. Substrates 12 and 16 are then heated to thermally activate the plane; thereby cleaving the protrusion base portions in two. For more details on this approach, refer, for example, to M. Breul et al., “Smart-Cut: A New Silicon on Insulator material technology based on Hydrogen Implantation and Wafer Bonding,” (Proceedings 1996 International Conference on Solid State Devices and Materials, Japan, Journal of Applied Physics, Part 1, Vol. 36, no. 3B, pp. 1636-41, 1996) which is incorporated herein by reference. Regardless of the chosen separation technique, the resulting structure resembles FIG. 4, which shows top portions 15 a and 15 b of the semiconductive protrusions bonded to insulative carrier substrate 18.

FIG. 5 shows that the next steps of the exemplary method entails planarizing the bonded protrusions and forming them into respective transistors 15 a′ and 15 b′. The exemplary method applies conventional chemical-mechanical polishing to planarize the top (broken) surfaces of the bonded protrusions. However, a variety of other planarization techniques are available. Some entail doping the protrusions to form etch stops and then etching the protrusions using chemical and plasma-assisted processes. Some embodiments apply these known procedures to reduce the bonded protrusions to ultra-thin semiconductive layers with thicknesses of 100 nanometers or less.

The transistors formed from the bonded protrusions may be of any type. For example, in some embodiments, the transistors are field-effect transistors, floating-gate transistors, bipolar junction transistors, or combinations of two or more of these transistors as in BiCMOS circuits. In fact, the protrusions may be formed into any device or component which can be made from semiconductive material.

The exemplary method forms top portions 15 a and 15 b into field-effect transistors 15 a′ and 15 b′ by treating them as wells in a conventional CMOS process. Accordingly, the portions are doped via ion implantation to form n- or p-type drain and source regions 20 d and 20 s and a gate insulation layer 22 is deposited or grown over and between the doped protrusions. Notably, the application of oxide or other insulative material over and the between the doped protrusions not only highlights the absence of parasitic devices between the transistors, but also illustrates that the selective, or intelligent, bonding of semiconductive material only where devices are desired simplifies and improves device isolation by omitting material that could form parasitic devices such as transistors and diodes. Parasitic devices compromise reliability by promoting transistor latch-up and other component failures. In contrast, conventional wafer-bond methods, for example those addressed in the Background, indiscriminantly bond silicon to the entire surface on an insulative substrate, fostering parasitic device formation and potentially requiring additional, more costly isolation techniques to achieve isolation comparable to the present “smart-bonding” technique.

After forming gate insulation layer 22, the method forms gates 24 a and 24 b and interconnects them via a conductor 25. The gates and conductor are metal or polysilicon or a combination. Formation of an insulative layer 26 completes fabrication of the structure shown in FIG. 6, which for sake of clarity omits typical drain, source, and body contacts. The structure resembles that of a typical integrated circuit having a single level of components and one or more levels of interconnections.

FIG. 7 shows that the basic smart bonding technique for forming semiconductor-on-insulator structures is useful for forming an integrated circuit having two or more circuit levels, with each circuit level including one or more active or passive components and associated interconnections. However, prior to repeating the smart bonding technique with insulative layer 26 serving as a new insulative carrier substrate, semiconductive donor substrate 12 undergoes planarization and protrusion-formation procedures (similar to those already described) to produce a second set of protrusions 28, of which protrusions 28 a and 28 b are representative . Since each sequence of protrusion formation, protrusion separation, and preparatory planarization operations removes only a few microns of semiconductive material, donor substrate 12 may be reused hundreds or even thousands of times. The protrusions are then bonded to insulative layer 26.

Subsequently, the exemplary method, in accord with the procedures outlined for FIGS. 4-6, entails separating the bonded top portions 29 a and 29 b of protrusions 28 a and 28 b from substrate 12, forming them into transistors 29 a′ and 29 b′, and then interconnecting them through conductor 35, as shown in FIG. 8. Other structures formed during this series of steps include drain and source regions 30 d and 30 s, gate insulation layer 32, and gates 34 a and 34 b. The resulting “three-dimensional” integrated circuit includes two levels of interconnected components.

Exemplary Embodiment of a Three-Dimensional Integrated Memory Circuit Incorporating Silicon-on-Insulator Structures

FIG. 9 shows one example of the unlimited number of integrated circuits which would benefit from incorporation of the silicon-on-insulator structures of the present invention: a generic integrated memory circuit 40. Memory circuit 40, which operates according to well-known and understood principles, is generally coupled to a processor (not shown) to form a computer system. More particularly, circuit 40 includes a memory array 42 which comprises a number of memory cells 43, a column address decoder 44, and a row address decoder 45, bit lines 46, word lines 47, and voltage-sense-amplifier circuit 48 coupled to bit lines 46.

In the exemplary embodiment, the memory cells, the address decoders, and amplifier circuit are formed as silicon-on-insulator devices. Moreover, in other embodiments, certain components, for example, decoders 44 and 45 and amplifier 48, are formed on a different level than memory array 42, not only to minimize the average distance between the cells of array 42 and these components but also to allow for more cells on a given circuit level. Further embodiments even provide three-dimensional, that is, multi-level memory arrays, with each level of the array having its own decoders and amplifier circuits.

CONCLUSION

In furtherance of the art, the inventor has presented a smart-bond technique for making silicon-on-insulator, more generally semiconductor-on-insulator structures. Unlike conventional bonding techniques that covered entire insulative surfaces with silicon, the smart-bond technique attaches semiconductive material only where semiconductive devices are desired, avoiding the buckling and cracking that can occur with continuous silicon coverage, while also achieving better device isolation. In addition, the smart-bond technique allows repeated use of a single semiconductive donor wafer to produce hundreds, even thousands, of semiconductor-on-insulator wafers. Moreover, the smart-bond technique allows fabrication of three-dimensional integrated circuits.

The embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the invention, is defined only by the following claims and their equivalents. 

What is claimed is:
 1. An integrated circuit assembly comprising: two or more interconnected transistors; an insulative layer overlying and contacting the two or more transistors; and a semiconductor structure having two or more protrusions, with each of the protrusions having a top portion contacting the insulative surface and each protrusion separated from an adjacent protrusion by an open space.
 2. The integrated circuit assembly of claim 1, wherein the protrusions are noncontiguous.
 3. The integrated circuit assembly of claim 1, wherein each of the protrusions further comprises a base portion which is more narrow than the top portion in a dimension parallel to a major surface of the insulative layer.
 4. The integrated circuit assembly of claim 1, wherein the top portion of each of the protrusions has a rectangular cross-section in a plane parallel to a major surface of the insulative layer.
 5. The integrated circuit assembly of claim 1: wherein each of the protrusions further comprises a base portion which is more narrow than the top portion in a dimension parallel to a major surface of the insulative layer; and wherein the top portion has a substantially rectangular cross-section in a first plane parallel to a major surface of the insulative layer and the bottom portion has a non-rectangular cross-section in a second plane parallel to the first plane.
 6. The integrated circuit assembly of claim 1, wherein the protrusions are integral with the semiconductive structure.
 7. The integrated circuit assembly of claim 1, wherein each protrusion is integral with the semiconductive structure and its top portion has length and width dimensions less than those of a lowest portion of the semiconductive structure.
 8. An integrated circuit assembly comprising: two or more interconnected transistors; an insulative layer overlying and contacting the two or more transistors; and a semiconductor structure having two or more protrusions, with each of the protrusions having a top portion contacting the insulative surface and a lower portion having a width less than that of the top portion, and with each protrusion separated from an adjacent protrusion by an open space.
 9. The integrated circuit assembly of claim 8, wherein the semiconductive structure consists essentially of gallium-arsenic, germanium, silicon carbide, or silicon.
 10. The integrated circuit assembly of claim 8, wherein the protrusions are noncontiguous.
 11. An integrated circuit assembly comprising: two or more interconnected transistors; an insulative layer overlying and contacting the two or more transistors; and a semiconductor structure having two or more protrusions, with each of the protrusions having a top portion contacting the insulative surface and a lower portion having a thermally activatable cleavage plane.
 12. The integrated circuit assembly of claim 11, wherein the semiconductive structure consists essentially of gallium-arsenic, germanium, silicon carbide, or silicon.
 13. The integrated circuit assembly of claim 11, wherein the protrusions are noncontiguous.
 14. The integrated circuit assembly of claim 11, wherein the thermally activatable cleavage plane comprises implanted hydrogen ions.
 15. The integrated circuit assembly of claim 11: wherein each of the protrusions further comprises a base portion which is more narrow than the top portion in a dimension parallel to a major surface of the insulative layer; and wherein the top portion has a substantially rectangular cross-section in a first plane parallel to a major surface of the insulative layer and the bottom portion has a non-rectangular cross-section in a second plane parallel to the first plane.
 16. An integrated circuit assembly comprising: a first insulative structure having a surface; two or more transistors on the surface of the first insulative structure; at least one first conductor electrically coupling two or more of the transistors; a second insulative structure overlying the one first conductor and having a surface; and a semiconductor structure having two or more protrusions, with each of the protrusions contacting the surface of the second insulative structure and having a thermally activatable cleavage plane.
 17. The integrated circuit assembly of claim 16, wherein the semiconductive structure consists essentially of gallium-arsenic, germanium, silicon carbide, or silicon.
 18. The integrated circuit assembly of claim 16, wherein the protrusions are noncontiguous.
 19. The integrated circuit assembly of claim 14, wherein the thermally activatable cleavage plane comprises implanted hydrogen ions.
 20. The integrated circuit assembly of claim 16: wherein each of the protrusions further comprises a base portion which is more narrow than the top portion in a dimension parallel to a major surface of the insulative layer; and wherein the top portion has a substantially rectangular cross-section in a first plane parallel to a major surface of the insulative layer and the bottom portion has a non-rectangular cross-section in a second plane parallel to the first plane.
 21. An integrated circuit assembly comprising: a first insulative structure having a surface; two or more transistors on the surface of the first insulative structure; at least one first conductor electrically coupling two or more of the transistors; a second insulative structure overlying the one first conductor and having a surface; and a semiconductor structure having two or more protrusions, with each of the protrusions having a top portion contacting the surface of the second insulative structure and each protrusion separated from an adjacent protrusion by an open space.
 22. The integrated circuit assembly of claim 21, wherein the semiconductive structure consists essentially of gallium-arsenic, germanium, silicon carbide, or silicon.
 23. The integrated circuit assembly of claim 21, wherein each insulative structure consists essentially of silicon dioxide.
 24. The integrated circuit assembly of claim 21, wherein the protrusions are noncontiguous.
 25. The integrated circuit assembly of claim 21: wherein each of the protrusions further comprises a base portion which is more narrow than the top portion in a dimension parallel to a major surface of the insulative layer; and wherein the top portion has a substantially rectangular cross-section in a first plane parallel to a major surface of the insulative layer and the bottom portion has a non-rectangular cross-section in a second plane parallel to the first plane.
 26. The integrated circuit assembly of claim 21, wherein each of the protrusions comprises drain and source regions.
 27. An integrated circuit assembly comprising: a first insulative structure having a surface; two or more transistors on the surface of the first insulative structure; at least one first conductor electrically coupling two or more of the transistors; a second insulative structure overlying the one first conductor and having a surface; and a semiconductor structure having two or more protrusions, with each of the protrusions having a top surface contacting the surface of the second insulative structure and a portion having a thermally activatable cleavage plane.
 28. The integrated circuit assembly of claim 27, wherein the semiconductive structure consists essentially of gallium-arsenic, germanium, silicon carbide, or silicon.
 29. The integrated circuit assembly of claim 27, wherein each insulative structure consists essentially of silicon dioxide.
 30. The integrated circuit assembly of claim 27, wherein the protrusions are noncontiguous.
 31. The integrated circuit assembly of claim 27: wherein each of the protrusions further comprises a base portion which is more narrow than the top portion in a dimension parallel to a major surface of the insulative layer; and wherein the top portion has a substantially rectangular cross-section in a first plane parallel to a major surface of the insulative layer and the bottom portion has a non-rectangular cross-section in a second plane parallel to the first plane.
 32. The integrated circuit assembly of claim 27, wherein the thermally activatable cleavage plane comprises implanted hydrogen ions.
 33. An integrated circuit assembly comprising: two or more interconnected transistors; an insulative layer overlying and contacting the two or more transistors; and one or more semiconductive structures on the insulative layer, with the insulative layer between the semiconductive structures and the two or more transistors, wherein each of the semiconductive structures comprises a wide portion contacting the insulative layer and a narrow portion extending from the wide portion and each semiconductive structure is separated from an adjacent semiconductive structure by an open space.
 34. The integrated-circuit assembly of claim 33, wherein one or more of the semiconductive structures consists essentially of gallium-arsenic, germanium, silicon carbide, or silicon.
 35. The integrated-circuit assembly of claim 33, wherein the insulative layer consist essentially of silicon dioxide.
 36. The integrated circuit assembly of claim 33, wherein the wide portion has a rectangular cross-section in a plane parallel to a major surface of the insulative layer. 